1. Field of the Invention
The present invention relates to a frequency converter for an electric motor having a rectifier, an intermediate circuit, an inverse rectifier, and a control and regulation circuit.
2. Description of the Prior Art
The application of frequency converters for electric motors is of increasing importance for motors of a smaller power. The manufacturing costs for the frequency converter always still form a decisive criterium for its application. One therefore strives to simplify the construction of the frequency converter or to select this construction such that components may be used which are as inexpensive as possible.
With frequency converters for electric motors one differentiates essentially between two construction types, specifically the pulse-width-modulated frequency converter (PWM frequency converter) and the pulse-amplitude-modulated frequency converter (PAM frequency converter). Both not only change the frequency of the voltage lying at the motor but also the voltage itself to control the rotational speed of the motor with low losses and to be able to operate the motor with an essentially constant torque, specifically the motor voltage and motor frequency must be changed proportionally.
PAM Frequency converters consist essentially of a rectifier in which a single phase or multi-phase alternating current is rectified, of an intermediate circuit (also called direct voltage intermediate circuit) in which the constant direct voltage of the rectifier by way of an electronic switch is switched on and off in order thus to produce a direct current changeable with respect to magnitude, of an inverse rectifier and of a control and regulation circuit. In order to change the constant direct voltage lying at the exit of the rectifier, in particular to also to increase this, within the intermediate circuit there is provided a so-called Buck-Boost converter which contains a Buck-Boost rectifier diode as well as the already mentioned electronic switch. In the inverse rectifier then the variable direct voltage produced in the intermediate circuit by way of electronic switches is connected to the motor windings such that by way of a suitable control activation of the switches a frequent voltage supply of the motor is achieved, wherein by way of the selection of the frequency the rotational speed of the motor is controlled. With this to each motor phase there are allocated two electronic switches. One of the switches is connected to the higher potential (High-Side) and the other to the lower potential (Low-Side) in order thus for the respective motor phase to produce an alternating voltage. The control of the switches in the inverse rectifier as well as of the switch in the intermediate circuit is effected by way of the control and regulation circuit which with known PAM frequency converters is quite costly. The control is costly because the switch of the Buck-Boost converter as well as usually at least the n-type doping transistors sitting on the High-Side of the inverse rectifier are so seated in the voltage-conducting leads to the motor that their reference voltage constantly changes corresponding to the motor control activation. Since for switching these electronic switches a control voltage is required which lies above the reference voltage by a defined amount, in the control and regulation circuit for each of these switches the voltage must be suitably followed which is expensive with regard to circuit technology.
Against this background it is the object of the invention to provide frequency converter having reduced manufacturing cost over the prior art.
According to the present invention, the object is achieved by a frequency converter for an electric motor including a rectifier, an intermediate circuit having a buck-boost converter, an inverse rectifier and a control circuit. The intermediate circuit receives an output of the rectifier. The inverse rectifier has a high-side conducting a higher potential and a low-side conducting a lower potential and a pair of electronic switches for each phase of the electric motor. One of the pair of switches is connected to the high side and the other of the pair of switches is connected to the low side.
The basic concept of the invention is to control the intermediate circuit such that the output voltage of the intermediate circuit is always smaller than the output voltage of the rectifier.
Such a control activation according to the present invention is contrary to the known intermediate circuit, because the known intermediate circuit with a Buck-Boost converter indeed is applied exactly for the reason of increasing the output voltage of the intermediate circuit also beyond the output voltage of the rectifier. This control activation according to the present invention however permits the application of inexpensive electronic switches and beyond this permits a considerably simpler construction of the control and regulation circuit.
The solution according to the present invention permits the switches of the inverse rectifier, on the side conducting the higher potential to comprise p-type doping semiconductors and on the side conducting the lower potential to comprise n-type doping semiconductors, with known inverse rectifiers according to the state of the art only n-type doping semiconductors are applied since specifically the p-type doping transistors may only be applied up to approx. 150 Volts. The n-type doping semiconductors which are also applied on the High-Side in the known inverse rectifier not only have the disadvantage that they are significantly more expensive than the p-type doping but with the application on the High-Side the n-type doping semiconductors have the disadvantage that they are connected to the further motor winding such that their reference voltage constantly changes which has the result that the voltage required for the switching must be correspondingly followed. The present invention thus permits the application of inexpensive p-type doping semiconductors on the High-Side of the inverse rectifier which furthermore considerably simplifies the control actuation of these electronic switches, since no voltage following for the turn-on voltage is required. This applies in a similar manner to the switch of the Buck-Boost converter which is why it is particularly advantageous when this electronic switch of the Buck-Boost converter is arranged one side of the intermediate circuit and the Buck-Booster diode usually arranged in the Buck-Booster converter is arranged on the other side of the intermediate circuit. Then specifically it is possible to control activate the switches in a simple manner with a constant turn-on voltage, wherein the switch preferably lies on the side of the Buck-Boost converter which impinges the Low-Side at the output, and the diode on the other side, which forms the High-Side at the output. The control and regulation circuit may then thus likewise be constructed relatively simply for this switch.
The application of a Buck-Boost converter in the intermediate circuit firstly appears to be contradictory when the voltage at the exit of the rectifier is not to be increased at all but if anything is to be reduced in magnitude. This voltage reduction, apart from the above mentioned advantages, has a very considerable further advantage which in particular comes to fruition when the frequency converter is envisaged for motors of a lower power, i.e. between for example 50 and 300 Watts. If such motors are designed for a frequency converter operation, i.e. wound as for example three-phase asynchronous motors, then one meets problems with regard to manufacturing technology on account of the then resulting extremely thin wire thicknesses which likewise may be compensated with the frequency converter according to the invention. Since on account of the low voltage with the optimization of the present motor for this frequency converter with the same power, there must flow a larger current, the wire thickness must also be selected larger which in this particular case is advantageous.
Furthermore the voltage reduction in the intermediate circuit also has the advantage that the components in the intermediate circuit and inverse rectifier as well as in the control and regulation circuit instead of the usual 400 Volts must only have a disruption strength of for example 50 Volts, which means the application of less expensive and smaller components. Thus for example also the passive components such as capacitors, coils and resistances may be designed less expensively.
A further advantage of the frequency converter according to the invention in particular to be seen on the part of the motor manufacturer is that one and the same apparatus may be operated with mains supply of a differing supply voltage and/or frequency. Also an operation with a direct current mains supply is possible which for example permits the application in vehicles and boats. Thus the frequency converter according to the invention is preferably also applicable in combination with a pulse-amplitude-modulated controlled, brush less, direct current motor for driving a centrifugal pump. The application of the Buck-Boost converter, unusual already for reasons of cost, in the intermediate circuit furthermore has the advantage that a continuous power output of the frequency converter is effected at the motor, i.e. that the interruptions in the power uptake otherwise occurring with alternating current motors do not occur with the zero crossing. Furthermore the switch of the Buck-Boost converter may advantageously also be used for suppressing the idle current and idle power, which in particular is already prescribed with motors of a smaller power today in many countries.